Particularly in the third generation (3G) mobile communications systems, a public land mobile network (PLMN) infrastructure may be logically divided into a core network (CN) and an access network (AN) infrastructures, as illustrated in FIG. 1. The access network AN may be called a base station subsystem (BSS) for GSM and a radio network subsystem (RNS) or a radio access network (RAN) for UMTS. In the technical specifications of a third generation partnership project (3GPP), the core network CN is logically divided into a circuit switched (CS) domain, a packet switched (PS) domain and an IP multimedia subsystem (IMS). The CS domain refers to a set of all the CN entities offering “CS type of connection” for user traffic as well as to all the entities supporting the related signaling. A “CS type of connection” is a connection for which dedicated network resources are allocated at the connection establishment and released at the connection release. A “PS type of connection” transports the user information using packets so that each packet can be routed independently of the previous one. An example of the PS domain is the GPRS (General Packet Radio Service), and the typical entities may include a serving GPRS support node (SGSN) and a gateway GPRS support node (GGSN). The IP multimedia subsystem comprises all CN elements for provision of multimedia services. The IP multimedia subsystem IMS utilizes the PS domain to transport multimedia signaling and bearer traffic.
Push-to-talk over Cellular (PoC) is an overlay speech service in a mobile cellular network where a connection between two or more parties is established (typically) for a long period but the actual radio channels in the air interface are activated only when somebody is talking. This corresponds to the usage of the traditional radio-telephones where the radio frequency used is agreed on between the parties (e.g. military/police radios, LA radios) or permanently set (walkie-talkie type of radios), and whenever someone wishes to talk, s/he presses the tangent which activates the radio transmission in the selected channel. The traditional radiotelephone services are simplex by nature so that only one party (the one who is pressing the tangent) can talk at a time. More specifically, in a voice communication with a “push-to-talk, release-to-listen” feature, a call is based on the use of a pressel (PTT, push-to-talk switch) in a telephone as a switch: by pressing a PTT the user indicates his desire to speak, and the user equipment sends a service request to the network. Alternatively, a voice activity detector (VAD) or any suitable means can be used instead of the manual switch. The network either rejects the request or allocates the requested resources on the basis of predetermined criteria, such as the availability of resources, priority of the requesting user, etc. At the same time, a connection is established also to a receiving user, or users in the case of group communication. After the voice connection has been established, the requesting user may talk and the other users may listen. When the user releases the PTT, the event is detected in the network, and the resources are released and/or a speech item is granted to another user. Thus, the resources are reserved only for the actual speech transaction or speech item, instead of reserving the resources for a “call”.
Modern cellular networks, especially in the GSM/GPRS/UMTS network evolution, include new packet-mode (e.g. TP) voice and data services. A Push-to-talk over Cellular (PoC) service can be provided as a packet-based user or application level service so that the underlying communications system only provides the basic connections (i.e. IP connections) between the group communications applications in the user terminals and the group communication service. The PoC communication service can be provided by a communication server system while the client applications reside in the user equipment or terminals. Examples of this approach are disclosed in co-pending U.S. patent applications Ser. Nos. 09/835,867; 09/903,871; and 10/160,272; and in WO 02/085051.
With the PoC service, first the connection(s) between the parties is established typically via the packet switched (PS) mobile network, e.g. a packet switched (PS) core network. In practice, this means that a Voice over IP (VoIP; group or one-to-one) call is set up between the parties. However, as described above, the difference from a conventional phone call is that the radio channel of the subscribers is activated only when someone needs to talk and released when no one is talking.
The PoC service is a practical solution for cases wherein the parties need to talk relatively rarely but whenever someone needs to talk, the connection has to be activated fast and easily (e.g. when giving instructions to the members of a hunting team in the forest or to a crane driver on a construction site). Because in this type of applications the calls are typically long but the voice activity is low, it is essential to release the bearer (e.g. radio channels) while no one is talking in order to save the radio and network capacity and terminal batteries. On the other hand, the bearer resources should be available with as small a delay as possible when the voice activity starts again.